Stormwater runoff includes the initial runoff or “first flush” that contains sediments, oil, and other pollutants flushed from surface areas, and other runoff that can be considered essentially pollution-free. The pollution-free runoff includes the later runoff from the surface areas that generated the first flush, and runoff from areas without surface pollutants. In major storm events the volume of non-first flush is substantially greater than the volume of first flush.
Stormwater treatment systems have been developed to remove pollutants from the first flush. Conventional first flush treatment systems include systems that pass the first flush through a filter to remove pollutants. The filter can be a relatively inexpensive low-head filter because of the relatively low volume and flow of runoff to be filtered.
Underground stormwater management systems have also been developed that receive runoff at a high rate during a major storm event, and discharge the runoff at a lower rate to a storm drain. Such systems include an underground storage chamber that receives and stores the water that accumulates while the flow into the storage chamber is greater than the discharge out. The water discharges at a relatively high head from the storage chamber to enable discharge near the maximum discharge rate allowed by applicable law or regulation. The discharge is normally not filtered, but if filtering is desired an expensive high-head filter must be used because of the high volume and flow of runoff being filtered.
Sites such as shopping centers, business parks, and other developed areas often use separate stormwater treatment systems and underground stormwater management systems. The stormwater treatment system is connected to surface areas that generate relatively low volume first flush, while the underground stormwater management system is capable of receiving and accumulating a large volume of non-first flush runoff from major storm events. Building and maintaining two stormwater systems is expensive and can be difficult to locate on some sites.
Thus there is a need for an improved underground storage system that can receive both receives and filters first flush and stores and accumulates large amounts of non-first flush runoff during major storm events without the need for an expensive, high-head filter.